Newt Gingrich has surged to the top of most polls in the race for the GOP presidential nomination, despite being married three times — once more than President Obama and Mitt Romney combined. Herman Cain’s campaign, meanwhile, stumbled after allegations of sexual misconduct and crashed and burned after allegations of infidelity.

So what are voters saying about how much a candidate’s private life affects his public service? If the message is mixed, it’s because this campaign is playing out as our ideas about marriage are fundamentally changing. Still, the Republican race so far has confirmed three unwritten rules of love and marriage — some old, some new — that candidates must follow.

Rule 1. Be married.

This may seem like a no-brainer, but marriage matters much more here than it does in other Western countries.

In France, Ségolène Royal, the unsuccessful socialist candidate for president in 2007, had four children with a long-term partner, politician François Hollande, whom she never married. They announced a separation soon after the election, but the fact that they were unwed was not a political liability for her.

In Britain, Ed Miliband married the mother of his two children in May, less than a year after becoming the leader of the Labor Party. Miliband, who is likely to run for prime minister in the next election, previously responded to criticism about cohabiting by saying he was “too busy” to get married. The wedding, he said, had nothing to do with politics. Rather, he said, “it feels like the right time for us.”

In the United States, however, candidates must be married. It’s difficult to imagine a modern presidential hopeful cohabiting with his or her partner and their children. According to a 2010 Pew Research Center survey, 20 percent of Americans do not even think that a cohabiting couple with children constitutes a family. A new analysis of census data by the Pew Center showed that marriage rates have fallen dramatically in this country, while people’s attitudes about what counts as a family — and what they expect from their political leaders — are still evolving.

Americans seem to feel better about a candidate when they see the de rigueur image of a husband and wife smiling and waving together. The only never-married presidential candidate of any significance in recent memory is independent Ralph Nader, who has never come close to winning.

Rule 2. It doesn’t have to be your first marriage.

A divorce, perhaps even two, is not a problem. But as recently as the 1960s, the electorate would not accept a divorced and remarried candidate. In 1963, New York Gov. Nelson Rockefeller’s bid for the 1964 Republican nomination effectively ended when he divorced his wife and married Margaretta “Happy” Murphy, herself divorced.

America’s divorce rate nearly doubled in the 1960s and 1970s, to a level where nearly half of all marriages ended in divorce, and Americans’ attitudes began to change. In 1980, Ronald Reagan became the first person to be elected president who had divorced and remarried. If Gingrich were to win the Republican nomination, he would be the fifth major-party nominee to have been divorced and remarried, following Reagan, Bob Dole, John Kerry and John McCain.

Gingrich’s two splits and three marriages may actually be less of an issue in traditionally Republican states than in traditionally Democratic ones. That’s because in recent presidential elections, states that the Republican candidate won had higher divorce rates than states that the Democratic candidate won. In 2008, John McCain carried eight of the 10 states with the highest divorce rates, and in 2004 George W. Bush carried all 10.

Moreover, many evangelical churches, where the populations are often GOP-leaning, are opposed to divorce but willing to minister to divorced congregants. (If people show regret for their divorces and recommit themselves to God, conservative churches will take them in and help them heal.) If Gingrich is seen as sincerely repenting for his past, he may find that two divorces aren’t a problem among churchgoing voters.

He had the language down perfectly at the Dec. 10 debate in Iowa. Asked about his family life and how it reflected on him as a candidate, Gingrich said: “I’ve made mistakes at times; I’ve had to go to God for forgiveness; I’ve had to seek reconciliation.” Two days later, he signed on to a conservative organization’s pro-traditional-marriage, antiabortion platform, adding, “I also pledge to uphold the institution of marriage through personal fidelity to my spouse and respect for the marital bonds of others.”

Rule 3. Don’t have an affair.

Extramarital affairs, especially those uncovered in the course of a campaign, are still a problem with American voters. Cain, who was polling well in the GOP race this fall, saw his campaign falter because of sexual harassment allegations. But he ultimately suspended it because of an alleged extramarital affair.

That it was charges of an affair, rather than of harassment, that delivered the knock-out punch fits with what we know about public opinion. While attitudes toward sexual harassment are still in flux, Americans are strongly against extramarital sex. Indeed, they are more strongly against it now than in the recent past. In the General Social Survey, a national poll of adults conducted biennially by the National Opinion Research Center at the University of Chicago, the percentage of Americans who responded that it is “always wrong” for a married person to have sex with someone other than his or her spouse rose from 73 percent in 1991 to 81 percent in 2008.

Meanwhile, opinions about other aspects of sexual and married life have become more liberal. For instance, the percentage of people who responded that premarital sex is “not wrong at all” increased from 41 percent to 51 percent during the same period.

Put together, these three rules suggest that America is moving away from the old standard of lifelong monogamy to a new one of serial monogamy. Being married remains important, but we are allowed, even expected, to move from one marriage to another. However, we are supposed to remain sexually faithful to whomever we are married to at the time.

The problem with this new ethic is that it treats divorce as though it were an easy separation in which a faithful partner wearies of a spouse and wants to start searching for someone better. In practice, ending a marriage is often a gut-wrenching process. And it often involves extramarital affairs. In one national survey, 39 percent of those who had divorced or separated in the previous five years said that their spouses were involved with someone else before the marriage ended.

What we accept from our politicians in their personal lives is inconsistent with how our own personal lives work. The contradictions reflect our difficulty in coming to terms with the great changes in sex and marriage since our parents’ and grandparents’ generations. We value marriage, but we also value the right to pursue personal happiness.

To be consistent in what we are telling candidates, we would need to either return to the lifelong-monogamy standard and reject any candidate who has ever divorced, or suspend judgment on all aspects of a candidate’s personal life. The first is what we used to do in America; the second is what the French do today.

Neither alternative is attractive. A one-divorce-and-you’re-out rule seems too restrictive today. Yet turning a blind eye to personal life may prevent us from making fully informed judgments about a candidate’s integrity.

Perhaps the best we can do is emulate the British — give candidates wider latitude in the way they organize their family lives, but recognize the symbolic importance of markers of family stability, such as marriage. Private life would be neither fully on nor fully off the table. Voters would continue to expect political leaders to show moral worth in crafting policy and law. They would consider a candidate’s marital history if it were relevant to his or her conduct in office. But they wouldn’t hold the love lives of politicians to a higher standard than the love lives of the people they represent.

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